This invention is directed to a composition useful for coating a variety of substrates. In particular, this invention is directed to the use of catalysts for curing organosilane compositions, especially compositions which are hydrolytically stable.
It is well known that consumers prefer automobiles and trucks with an exterior finish having an attractive aesthetic appearance, including high gloss and excellent DOI (distinctness of image). While ever more aesthetically attractive finishes have been obtained, deterioration of the finish over time, whereby the exterior finish of an automobile or truck loses its luster or other aspects of its aesthetic appearance, may be all the more noticeable. An increasingly observed cause of this deterioration is etching of the finish caused by exposure to environmental chemical attack. Chemicals that may cause etching of a finish include pollutants such as acid rain and chemical smog.
In order to protect and preserve the aesthetic qualities of the finish on a vehicle, it is generally known to provide a clear (unpigmented) topcoat over a colored (pigmented) basecoat, so that the basecoat remains unaffected even on prolonged exposure to the environment or weathering. It is also generally known that alkoxysilane polymers, due to strong silane bonding when cured, exhibit excellent chemical resistance. Exemplary of prior art patents disclosing silane polymers for coating are U.S. Pat. Nos. 4,368,297; 4,518,726; 4,043,953; and Japanese Kokai 57-12058.
However, to applicants, knowledge, none of the previously disclosed alkoxysilane compositions for finishing automobiles or trucks have ever as yet been placed into commercial use. It is believed that heretofore known or patented alkoxysilane coatings have suffered from various unsolved problems or deficiencies. One problem with the use of such silane compositions is that if the composition becomes contaminated with water or moisture before it is applied, there is a danger the composition may prematurely gell, thereby becoming useless.
The premature gelling of a siloxane or silane polymer is related to its curing or crosslinking reactivity. There is a reactivity difference between various silane polymers. Two steps are believed involved in the cure mechanism. The first is the hydrolysis of the alkoxy silane groups in the silane polymer to give a more reactive silanol. Subsequent condensation ensues to give the siloxane crosslink. The complete mechanism as to how these two reactions occur is not fully understood. Typically, as one moves from methoxy to ethoxy to propoxy to butoxy silanes, the rate of hydrolytic cleavage decreases, due primarily to the increase in steric bulk around the silicon atom. For the latter reason, ethoxy silane and higher alkoxy silanes, as compared to methoxy silanes, have been referred to as "hydrolytically stable silanes." Although offering hydrolytic stability, such polymers are, however, more difficult to cure. Conventional curing catalysts for methoxy silanes are generally not sufficiently effective for hydrolytically stable alkoxy silanes.
Various catalysts have been identified for curing of silane polymers. Typically, water and heat have been used. A variety of mineral acids have found broad application, including H.sub.2 SO.sub.4, H.sub.3 BO.sub.3, H.sub.3 PO.sub.3 and the like. These catalysts are believed to promote catalysis by protonation of the alkoxy group with subsequent displacement of the ensuing alcohol by the nucleophile, water, giving the silanol. Other catalysts which have found widespread use include various Lewis acids. These are electron-deficient species which are able to coordinate to the oxygen of the alkoxy group attached to the silicon and thereby promote its displacement. The most widely used such catalyst is dibutyl tin dilaurate. Other conventional catalysts include amines, sulfonic acids, and blends of catalysts. Various conventional catalysts are disclosed in European Patent No. 0308203.
As indicated above, conventional curing catalysts, although effective for curing methoxy silanes, are generally ineffective for curing hydrolytically stable silanes. There is a need for catalysts which can enhance the curing of ethoxy silane and other hydrolytically stable silane compositions. If such catalysts were found, they might be useful for making a coating composition having an extended shelf life, which composition is not be susceptible to premature gelling. By means of such catalysts, it might be possible to obtain a one package silane composition.